Mariusz Radoń

Electronic Structure of Selected {FeNO}7 Complexes in Heme and Non-Heme Architectures: A Density Functional and Multireference ab Initio Study

The multiconfigurational CASSCF/CASPT2 approach, along with various functionals of density functional theory, is applied to selected iron(II)-nitrosyl ({FeNO}(7)) complexes, both with heme and nonheme groups. The energetics of the lowest doublet and quartet spin states at the correlated ab initio (CASPT2) level is presented for the first time. Comparison of the CASSCF and (unrestricted) DFT spin densities indicates that the nonhybrid functionals yield the spin densities most closely to the ab initio ones. The analysis of the multiconfigurational CASSCF wave function in terms of the localized active orbitals allows one to resolve the nature of Fe-NO bonding as a mixture of Fe(II)-NO(0) and Fe(III)-NO(-) resonance structures (in comparable contributions) for both spin states and various ligands.

Performance of CASPT2 and DFT for Relative Spin-State Energetics of Heme Models

The accuracy of the relative spin-state energetics of three small Fe(II) or Fe(III) heme models from multiconfigurational perturbation theory (CASPT2) and density functional theory with selected functionals (including the recently developed M06 and M06-L functionals) was assessed by comparing with recently available coupled cluster results. While the CASPT2 calculations of spin-state energetics were found to be very accurate for the studied Fe(III) complexes (including FeP(SH), a model of the active site of cytochrome P450 in its resting state), there is a strong indication of a systematic error (around 5 kcal/mol) in favor of the high-spin state for the studied Fe(II) complexes (including FeP(Im), a model of the active site of myoglobin). A larger overstabilization of the high-spin states was observed for the M06 and M06-L functionals, up to 22 and 11 kcal/mol, respectively. None of the tested density functionals consistently provides a better accuracy than CASPT2 for all model complexes.

Mechanism of Selective Halogenation by SyrB2: A Computational Study

The mechanism of the chlorination reaction of SyrB2, a representative α-ketoglutarate dependent halogenase, was studied with computational methods. First, a macromolecular model of the Michaelis complex was constructed using molecular docking procedures. Based on this structure, a smaller model comprising the first- and some of the second-shell residues of iron and a model substrate was constructed and used in DFT investigations on the reaction mechanism. Computed relative energies and Mössbauer isomer shifts as well as quadrupole splittings indicate that the two oxoferryl species observed experimentally are two stereoisomers resulting from an exchange of the coordination sites occupied by the oxo and chloro ligands. In principle both Fe(IV)═O species are reactive and decay to Fe(III)Cl (OH)/carbon radical intermediates via C-H bond cleavage. In the final rebound step, which is very fast and thus precluding equilibration between the two forms of the radical intermediate, the ligand (oxo or chloro) placed closest to the carbon radical (trans to His235) is transferred to the carbon. For the native substrate (L-Thr) the lowest barrier for C-H cleavage was found for an isomer of the oxoferryl species favoring chlorination in the rebound step. CASPT2 calculations for the spin state splittings in the oxoferryl species support the conclusion that once the Fe(IV)═O intermediate is formed, the reaction proceeds on the quintet potential energy surface.

DFT and Ab Initio Study of Iron-Oxo Porphyrins: May They Have a Low-Lying Iron(V)-Oxo Electromer?

The energetics of various electromeric states for two heme complexes with an iron-oxo (FeO(3+)) group, FeO(P)(+) and FeO(P)Cl (P = porphin), have been investigated, employing DFT and correlated ab initio methods (CASPT2, RASPT2). Our interest focused in particular on tri- and pentaradicaloid iron(IV)-oxo porphyrin radical states as well as iron(V)-oxo states. Surprisingly, the iron(V)-oxo ground state is predicted for both models in vacuo. However, the presence of a polarizable medium, such as a solvent or a protein environment, favors the iron(IV)-oxo porphyrin radical cation, which is predicted to be the actual ground state of FeO(P)Cl under such conditions. Nonetheless, the iron(V)-oxo electromer is still expected to lie only a few kcal/mol above the ground state-a conclusion coming from both CASPT2 and RASPT2 calculations with a very large active space and further supported by a calibration with respect to coupled cluster CCSD(T) calculations for a simplified small model. The DFT results turn out to be strongly functional-dependent and thereby inconclusive. The widely used B3LYP functional-although correctly predicting the iron(IV)-oxo porphyrin radical ground state for FeO(P)Cl-seems to place the iron(V)-oxo states much too high in energy, as compared to the present CASPT2, RASPT2, and CCSD(T) results.

Spin-State Energetics of Heme-Related Models from DFT and Coupled Cluster Calculations

Spin-state energetics of metalloporphyrins and heme groups is elucidated by performing high-level coupled cluster calculations for their simplified mimics. An efficient computational protocol is proposed-based on the mix of extrapolation to complete basis set and explicitly correlated (F12) methodology-which retains the high accuracy of the CCSD(T) method at a cost that makes it applicable also to relatively large models, e.g., FeP and FeP(Cl) (P = porphin). Adequacy of CCSD(T) is supported by analysis of multireference character and comparison with the completely renormalized CR-CC(2,3) method. The high-level coupled cluster results are used for assessment of density functional theory (DFT) methods, for which an accurate description of the spin-state energetics is recognized as a major challenge. Although the DFT results are highly functional-dependent, it is shown that the spin-state energetics of a full heme model and its simplified mimic remain in a good linear correlation. This makes it possible to estimate the spin-state energetics of full heme models based on the accurate CCSD(T) results for their mimics, as illustrated for porphyrin complexes of Fe(II), Mn(II), and Co(II); pentacoordinate heme complexes of Fe(II) and Fe(III); and a ferryl heme model. Comparison with the available experimental data is also presented.

Conformational Stability and Spin States of Cobalt(II) Acetylacetonate: CASPT2 and DFT Study.

Electronic structure and conformation of bis(acetylacetonate) cobalt(II), Co(acac)2, a prototypical mediator in controlled radical polymerization of olefins, is reinvestigated. The ab initio multiconfigurational CASSCF/CASPT2 method is used to resolve the doubts stemming from density functional theory results. We assign the quartet ground state for a single molecule and point at tetrahedral conformation as the preferred one. Several density functionals are tested against the ab initio calculations, and their performance is assessed. The strength of intermolecular interactions in the crystal structure composed of square-planar Co(acac)2 molecules ( Burgess , J. ; et al. Acta Crystallogr. 2000 , C56 , 649 - 650 ) is estimated to be sufficient for their planarization (suggested by Matyjaszewski , K. ; et al. Chem.-Eur. J. 2007 , 13 , 2480 - 2492 ).

Peculiarities of the Electronic Structure of Cytochrome P450 Compound I: CASPT2 and DFT Modeling†

CASSCF/CASPT2 ab initio formalism has been applied to a thiolate model of cytochrome P450 compound I. A2u ground state of porphyrin radical character was found in agreement with experimental results. A strong mixing of CASSCF reference states in multistate CASPT2 was observed, which is an interesting phenomenon, rare for the ground state near the equilibrium geometry. Details of the CASSCF/CASPT2 procedure (including the construction of the active space) are discussed. Parallel DFT calculations revealed that relative energies and the scheme of spin coupling are qualitatively reproduced by hybrid DFT (B3LYP); however, results from nonhybrid functionals (BLYP, BP86) are significantly different in these aspects.

Mechanism of O2 Activation by α-Ketoglutarate Dependent Oxygenases Revisited. A Quantum Chemical Study

Four mechanisms previously proposed for dioxygen activation catalyzed by α-keto acid dependent oxygenases (α-KAO) were studied with dispersion-corrected DFT methods employing B3LYP and TPSSh functionals in combination with triple-ζ basis set (cc-pVTZ). The aim of this study was to revisit mechanisms suggested in the past decade and resolve remaining issues related to dioxygen activation. Mechanism A, which runs on the quintet potential energy surface (PES) and includes formation of an Fe(III)-superoxide radical anion complex, subsequent oxidative decarboxylation, and O-O bond cleavage, was found to be most likely. However, mechanism B taking place on the septet PES involves a rate limiting barrier comparable to the one found for mechanism A, and thus it cannot be excluded, though two other mechanisms (C and D) were ruled out. Mechanism C is a minor variation of mechanism A, whereas mechanism D proceeds through formation of a triplet Fe(IV)-alkyl peroxo bridged intermediate. The study covered also full optimization of relevant minimum energy crossing points (MECPs). The relative energy of critical intermediates was also studied with the CCSD(T) method in order to benchmark TPSSh and B3LYP functionals with respect to their credibility in predicting relative energies of septet and triplet spin states of the ternary enzyme-Fe-α-keto glutarate (α-KG)-O2 complex.

On the nature of spin- and orbital-resolved Cu+–NO charge transfer in the gas phase and at Cu(I) sites in zeolites

Electronic factors essential for NO activation by Cu(I) sites in zeolites are investigated within spin-resolved analysis of electron transfer channels (natural orbitals for chemical valence). NOCV analysis is performed for three DFT-optimized models of Cu(I)–NO site in ZSM-5: [CuNO]+, (T1)CuNO, and (M7)CuNO. NO as a non-innocent, open-shell ligand reveals significant differences between independent deformation density components for α and β spins. Four distinct components are identified: (i) unpaired electron donation from NO π‖* antibonding orbital to Cus,d; (ii) backdonation from copper dyz to π⊥* antibonding orbital; (iii) donation from occupied π‖ and Cu dxz to bonding region, and (iv) donation from nitrogen lone-pair to Cus,d. Channel (i), corresponding to one-electron bond, shows-up solely for spin majority and is effective only in the interaction of NO with naked Cu+. Channel (ii) dominates for models b and c: it strongly activates NO bond by populating antibonding π* orbital and weakens the N–O bond in contrast to channel (i), depopulating the antibonding orbital and strengthening N–O bond. This picture perfectly agrees with IR experiment: interaction with naked Cu+ imposes small blue-shift of NO stretching frequency while it becomes strongly red-shifted for Cu(I) site in ZSM-5 due to enhanced backdonation.

Mono- and Dinitrosyls on Copper(I) Site in a Zeolite Model: Effects of Static Correlation

Multiconfigurational RASSCF/RASPT2 approach has been applied to investigate bonding of one and two nitric oxide (NO) molecules to a simple model of Cu(I) site in zeolite environment, Cu(I)[Al(OH)(4)]. Two binding modes were considered for the mononitrosyls and four alternative structures for the dinitrosyls (each one in either singlet or triplet state). Stabilities of the mono- and dinitrosyl complexes obtained from the multireference calculations were compared to the previously reported coupled cluster CCSD(T) results, as well as to DFT calculations performed here with various functionals, either hybrid or nonhybrid ones. RASSCF calculations provided also a qualitative insight into the electronic structure of the studied complexes, concerning mainly the interaction between the Cu and the NO ligand, and between the two NO fragments. Whereas the electronic structure of the mononitrosyls is dominated by a single configuration, the dinitrosyls have a considerably multireference character. Various effects of nondynamical correlation have been pointed out for these interesting species, trying to assess their impact on performance of the tested DFT methods.